I notice this trend mostly with guys. I suppose it comes with the male desire to categorize and compare things.
But ultimately, you don't gain much from these ratings. It's specific to that one person, and it depends on their definition of a "10", which can sometimes be within the realism range, or other times completely out the wall insane, making the average person a 3 or 4.
When I ask questions like this, it's usually to look for tips on how to improve my look, or to get an idea of what kinds of girls I currently attract. A lot of times if I ask for a number, I get a "7", and then they give an example of a 10 which is basically Kanye West with his shirt off. (By that standard, a 10 would be an insult, and I would want those particular girls to rate me pretty low.)
Most Helpful Girl
Because 10 is the furthest they can count.3